Paper-punching device for use in an image-forming apparatus

Abstract: In a paper-punching device (33) for use in an image-forming apparatus, the rear edge of a sheet of paper that is being transported through a transport guide is detected by the photosensor (32). After a predetermined time has passed since the detection, a punching device (33) is activated. At this time, a punching member (38) is depressed downward, and a punching blade (38a) attached to its top penetrates through the sheet of paper, thereby forming a punch hole. Even during the punching operation, transport rollers (34,35) continue to rotate. Therefore, when the sheet of paper is caught by the punching blade (38a), the transport rollers (34,35) are allowed to make a slip at a predetermined amount with respect to the paper. For this reason, at least one of the transport rollers (34,35) is made of a foamed material. With this arrangement, since the punching operation is carried out on the rear side of the sheet of paper, it becomes possible to reduce the occurrence of paper jams even if sheets of paper are transported in succession. Further, the punching operation is carried out while the paper is being transported; this makes it possible to provide a high-speed operation. Further, even if the sheet of paper is caught by the punching blade, it is not damaged because the slip is provided.

Description: FIELD OF THE INVENTION The present invention relates to a paper-punching device for use in an image-forming apparatus, which forms holes through sheets of paper that have been subjected to image-forming operations in an apparatus such as a copying machine. BACKGROUND OF THE INVENTION Some of conventional image-forming apparatuses are provided with a paper-punching device for forming holes through sheets of paper in order to improve the efficiency of jobs for sorting the sheets of paper that have been subjected to the image-forming operations into a set of documents. Moreover, in recent years, with the wide spread of office automation apparatuses for handling sheets of paper, such as used in copying machines for ordinary paper (PPC) and automatic paper feeders (APF), and with the trend to high-speed, highly-effective operations in those apparatuses, there have been also strong demands toward a fast, highly-efficient punching operation, which is required before filing sheets of paper that have been subjected to the copying operation. As one example for such an paper-punching device, Japanese Laid-Out Patent Publication No. 140755/1983 (Tokukaishou 58-140755) has disclosed a paper-punching device. The following description will discuss this paper-punching device. Here, for convenience of explanation, the following example has an arrangement that is slightly different from that of the above-mentioned patent publication. As illustrated in Fig. 25, in the above-mentioned paper-punching device, a sheet of paper P is transported by transport rollers 303 and 304 from the upstream side on a base 301 while it is restricted in its upward dislocation by a transport guide 302. When the leading edge of the sheet of paper P passes through a light path of a photosensor 305 of the reflection type, the leading edge of the sheet of paper P is detected by the photosensor 305. Then, a stopper 306, located on the downstream side of the photosensor 305, moves upward from its stand-by station, and presses the leading edge of the sheet of paper P, thereby stopping the transportation of the sheet of paper P. Immediately after the stoppage of rotation of the transport roller 304, a punching blade 307 is shifted down toward a punching die 308 that is provided in the base 301, and the sheet of paper P is thus punched by the punching blade 307. At this time, the transport roller 304, located on the downstream side, is stopped in its rotation, while the transport roller 303, located on the upstream side, is being rotated. Therefore, the sheet of paper P is transported by the transport roller 303 from the rear-edge side, and is warped inside a warp-space 302a that is provided in an upward-raised form between the transport roller 303 and the transport roller 304 in the transport guide 302. With this arrangement, the transportation of the sheet of paper P is not stopped completely. Further, since the pressing force of the stopper 306 exerted onto the sheet of paper P is increased, the sheet of paper P does not retreat even upon the punching operation. However, in the paper-punching device as described in the above-mentioned patent publication, the leading edge of the sheet of paper P is temporarily stopped even it is a short period of time. Therefore, when the operation speeds of the image-forming process and other related processes are increased beyond a certain limit, the next paper is transported although the proceeding paper has not been subjected to the punching operation; this causes troubles such as paper jams. Moreover, the sheet of paper P might be damaged when it is warped. Furthermore, if the sheet of paper P is thick paper weighing not less than 228 g/m 2 , it is not allowed to warp, thereby making it difficult to keep transporting sheets of paper P by the use of the transport roller 303 during the punching operation. Another problem is that it is not possible to form punch holes on the rear side of the sheet of paper P due to the structure of the paper punching device. Moreover, although not described in detail here, another paper-punching device for use in an electrophotographic apparatus has been known to the art, wherein a punching operation is carried out with a sheet of paper P completely stopped, and after the punching operation the transportation is resumed. In this type of paper-punching device, it is possible to install the punching mechanism either on the leading side or on the rear side of the sheet of paper P. However, since the punching operation is carried out after stopping the sheet of paper completely, it is impossible to increase the speed of the operation. Prior art document EP-A-0 409 204 discloses a paper-punching device for use in an image-forming apparatus which comprises guiding means for guiding a sheet of paper in a predetermined direction, a punching blade for forming a punch hole in the sheet of paper, driving means for driving the punching blade, and transport rollers for constantly carrying the sheet of paper. The punching blade is installed in the guiding means, and the transport rollers are installed on the downstream side from the punching blade in the guiding means. SUMMARY OF THE INVENTION It is an object of the present invention to provide a paper-punching device for use in an image-forming apparatus which enables a high-speed punching operation without the necessity of reducing the transporting speed of paper of stopping sheets of paper. To solve this object the present invention provides a paper-punching device as specified in claim 1. Preferred embodiments of the invention as described in the subclaims. A first paper-punching device for use in an image-forming apparatus of the present invention as defined in claim 1 is provided with: a guiding means for guiding a sheet of paper in a predetermined direction; a punching blade for forming a punch hole in the sheet of paper, the punching blade being installed in the guiding means; a driving means for driving the punching blade, and transport rollers for carrying the sheet of paper, the transport rollers being installed on the downstream side from the punching blade in the guiding means, the transport rollers being allowed to make a slip at a predetermined amount with respect to the sheet of paper when the sheet of paper is caught by the punching blade, the product of the time during which the sheet of paper is caught by the punching blade and the transporting speed of the transport rollers is set to a predetermined value. In the first paper-punching device, when a sheet of paper is transported by the transport rollers along the guiding means and the rear edge of the sheet of paper reaches a predetermined position, the driving means is activated. Therefore, punching holes are formed in the sheet of paper at positions a predetermined distance apart from the rear edge. Here, since the transport rollers rotate all the time, the transportation of the sheet of paper is continued even when it is punched by the punching blade. For this reason, if the punching operation takes long time even slightly, the sheet of paper is pulled by the transport rollers because it is caught by the punching blade. However, when the sheet of paper is caught by the punching blade, the transport rollers are allowed to make a slip at a predetermined amount with respect to the sheet of paper; this slip absorbs the pulling force that is exerted on the sheet of paper by the transport rollers. In particular, when one of the transport roller is made of rubber and the other transport roller is made of a foamed material, it is possible to provide a preferable slipping property. Moreover, in the case when the time during which the paper is caught by the punching blade is long, such as in the case of using thick paper, the damage to the sheet of paper caused by the punching blade is reduced by decreasing the transporting speed. In contrast, in the case when the time during which the paper is caught by the punching blade is short, such as in the case of using normal paper, the damage to the sheet of paper caused by the punching blade is not increased even if the transporting speed is increased. In other words, under the condition where the transport rollers are allowed to make a slip with respect to paper, the arrangement, wherein the product of the time during which the sheet of paper is caught by the punching blade and the transporting speed exerted by the transport rollers is set to a predetermined value, makes it possible to reduce the damage to the sheet of paper caused by the punching blade. The above-mentioned setting will be clarified by experiments shown in the embodiments that will be described later. In accordance with the first paper-punching device, the sheet of paper is transported on the downstream side from the punching blade; therefore, it is possible to eliminate warping in the sheet of paper and to eliminate damage to the sheet of paper caused by warping. Further, it is possible to form punch holes even in thick paper that is hardly warped. Moreover, since the punching blade is disposed on the upstream side from the transport rollers, the punching operation is carried out on the rear-edge side of the sheet of paper. When a stapling operation is carried out simultaneously with the punching operation, this arrangement allows both of the operations to be carried out on the rear-edge side of the sheets of paper, thereby improving the efficiency of the operations. A second paper-punching device for use in an image-forming apparatus of the present invention as defined in claim 7 is provided with: a plurality of punching blades for forming punch holes in the sheet of paper, the punching blades being installed in the guiding means with predetermined intervals along a straight line that tilts at a predetermined angle with respect to the direction orthogonal to the transporting direction; a plurality of driving means for driving the punching blades individually; and a driving circuit for activating the driving means successively, starting with the one closest to the rear edge of the sheet of paper, with predetermined time-intervals. In the second paper-punching device for used in an image-forming apparatus, the respective punching blades are driven by the individual driving means, but these driving means are controlled by a driving circuit as a whole. Therefore, this arrangement eliminates the necessity of installing driving circuits to the respective driving means individually, thereby making it possible to reduce the number of parts. In this case, the driving circuit, which is not allowed to activate the respective driving means at the same time, activates each driving means in succession with predetermined time-intervals. Here, the sheet of paper is being transported even during the punching operation; therefore, it is necessary to adjust the punching positions in the transporting direction in the case when the driving means are individually activated and the respective punching blades form punch holes at different times. For this reason, the punching blades are installed along a straight line that tilts at a predetermined angle with respect to the direction orthogonal to the transporting direction, and the driving means are activated in succession, starting with the punching blade closest to the rear edge of the sheet of paper. This arrangement makes it possible to eliminate misalignment of the punching positions in the transporting direction. Thus, the punch holes formed by the punching blades are aligned virtually in parallel with the rear edge of the sheet of paper. More preferably, the following arrangement may be adopted: In the case when the driving circuit activates the respective driving means successively, supposing that a time-interval T after activation of a certain driving means, the next driving means is activated, the punching position has an offset of VT with respect to the transporting direction within the time-interval T when the sheet of paper is being transported at the transporting speed V. Therefore, in order to align the respective punch holes along one straight line, each having a constant distance from the rear edge of the sheet of paper, the value obtained by dividing VT by the distance x between the punching blades in the direction orthogonal to the transporting direction, that is, the interval between the punch holes, should be equal to the tangent to the predetermined angle (). In other words, the relationship indicated by tan = V·T/x should be satisfied. Therefore, if the driving circuit activates the respective driving means in succession with the time-intervals T while satisfying T = x·tan/V , it becomes possible to form punch holes at the proper positions as described above. A third paper-punching device for use in an image forming apparatus of the present invention as defined in claim 9 is provided with: discrimination means for discriminating whether the sheet of paper is normal paper having a thickness not more than a predetermined thickness, or thick paper having a thickness exceeding the predetermined thickness; driving means having a first driving source for driving the punching blade with a driving force that is suitable for normal paper and a second driving source for driving the punching blade with a driving force that is suitable for thick paper; a selective control means for activating the first driving source when the discrimination means shows that the sheet Of paper is normal paper, while activating the second driving source when the discrimination means shows that the sheet of paper is thick paper; and a transport control means for activating the transport rollers during the activation of the first driving source, while stopping the transport rollers during the activation of the second driving source. In the third paper-punching device, when a judgement is made by the discrimination means as to whether the sheet of paper is normal paper, or thick paper, the first or second driving source is selected and driven by the selective control means in accordance with the result of the judgement. Thus, punch holes are formed by using the driving force that is suitable for the sheet of paper. Further, the transport control means activates the transport rollers during the activation of the first driving source, and stops the transport rollers during the activation of the second driving source; this makes it possible to provide a high-speed operation during the punching operation for normal paper. A fourth paper-punching device for use in an image-forming apparatus of the present invention as defined in claim 12 is provided with: an idle-driving control means for activating the driving means so that the punching blade is driven in a specific period of time when there is no paper at the punching position in the guiding means. In the fourth paper-punching device, the idle-driving control means activates the driving means so that the punching blade is driven in a specific period of time when there is no paper at the punching position in the guiding means. This arrangement makes it possible to prevent the punching blade from being held at the punching position in the guiding means due to an insufficient driving operation, as well as preventing paper jams that occur when the sheet of paper is improperly caught by the punching blade. In a preferable application of the fourth paper-punching device, a paper-detection means, such as an optical sensor, is provided to detect the presence or absence of paper at the punching position, and according to this detection, it is possible to recognize the specific period of time when there is no paper at the punching position. Further, in a more preferable application, the idle-driving control means is arranged to inform the fact that there is paper at the punching position; this makes it possible for the user to take necessary steps to correct a paper jam. For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. DESCRIPTION OF THE EMBODIMENTS Fig. 1 is an explanatory drawing that shows the internal structure of a copying machine that is commonly used in the respective embodiments of the present invention. Fig. 2 is an explanatory drawing that shows an operation section installed on the top of the copying machine of Fig. 1. Fig. 3(a) is a vertical cross-sectional view of a punching unit in the copying machine in accordance with the first embodiment of the present invention. Fig. 3(b) is a vertical cross-sectional view showing an operational state during a punching operation of the punching unit in the copying machine in accordance with the first embodiment of the present invention. Fig. 4 is a vertical cross-sectional view of the punching unit when seen from the downstream side of a transport guide. Fig. 5 is a front view showing arrangements of a piezoelectric element and a distortion-enlarging mechanism that are installed in a driving device in the punching unit. Fig. 6(a) is a plan view illustrating the shape of a punching blade that is not suitable for the punching unit. Fig. 6(b) is a front view illustrating the shape of the punching blade that is not suitable for the punching unit. Fig. 6(c) is a side view illustrating the shape of the punching blade that is not suitable for the punching unit. Fig. 7(a) is a plan view illustrating the shape of another punching blade that is not suitable for the punching unit. Fig. 7(b) is a cross-sectional view taken along the line A-A of Fig. 7(a) illustrating the shape of another punching blade that is not suitable for the punching unit. Fig. 8(a) is a plan view illustrating the shape of a punching blade that is suitable for the punching unit. Fig. 8(b) is a front view illustrating the shape of the punching blade that is suitable for the punching unit. Fig. 8(c) is a side view illustrating the shape of the punching blade that is suitable for the punching unit. Fig. 8(d) is a view when seen from a position making an angle of 45 degrees from the backward direction, illustrating the shape of the punching blade that is suitable for the punching unit. Fig. 9 is a block diagram showing a control system for controlling the operation of a punching device in the punching unit. Fig. 10 is a flow chart showing a sequence of punching processes that are carried out by the punching unit. Fig. 11(a) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 100 mm/sec, wherein rubber transport rollers were used. Fig. 11(b) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 200 mm/sec, wherein rubber transport rollers were used. Fig. 11(c) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 400 mm/sec, wherein rubber transport rollers were used. Fig. 12(a) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 100 mm/sec, wherein polyurethane-foam transport rollers were used. Fig. 12(b) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 200 mm/sec, wherein polyurethane-foam transport rollers were used. Fig. 12(c) is a histogram indicating the frequency of occurrence of troubles when punching operations were carried out by the punching unit at a transporting speed of 400 mm/sec, wherein polyurethane-foam transport rollers were used. Fig. 13(a) is an explanatory drawing that shows tears caused in a sheet of paper in experiments that were carried out in order to obtain the histograms of Figs. 11(a) through 11(c) as well as Figs. 12(a) through 12(c). Fig. 13(b) is an explanatory drawing that shows defective punch holes caused in a sheet of paper in the experiments. Fig. 13(c) is an explanatory drawing that shows scratches caused on a sheet of paper by the rollers in the experiments. Fig. 14 is a vertical cross-sectional view showing an arrangement of a punching unit that is used in the first modified example of the first embodiment of the present invention. Fig. 15 is a plan view showing an arrangement of a punching unit that is used in the second modified example of the first embodiment of the present invention. Fig. 16 is a plan view showing a sheet of paper that has been subjected to the punching operation by the punching unit of Fig. 15. Fig. 17 is a vertical cross-sectional view of a punching unit in the copying machine in accordance with the second embodiment of the present invention. Fig. 18 is a block diagram showing a control system for driving the punching unit of Fig. 17. Fig. 19 is a flow chart showing a sequence of selections for punching operations corresponding to respective modes that are carried out in the punching unit of Fig. 17. Fig. 20 is a front view showing the arrangement of essential parts in a copying machine in accordance with a modified example of the second embodiment of the present invention. Fig. 21 is a vertical cross-sectional view of a punching unit in the copying machine in accordance with the third embodiment of the present invention. Fig. 22 is a block diagram showing a control system for controlling the operation in the event of a paper jam in the punching device in the punching unit of Fig. 21. Fig. 23 is a flow chart showing a sequence of processes that are carried out during the warm-up of the copying machine having the punching unit of Fig. 21. Fig. 24 is a flow chart showing a sequence of processes that are carried out in order to monitor the completion of the punching operation of the punching unit in a modified example of the third embodiment of the present invention. Fig. 25 is a vertical cross-sectional view showing one example of an arrangement of a conventional paper-punching device. DESCRIPTION OF THE EMBODIMENTS [EMBODIMENT 1] Referring to Figs. 1 through 13, the following description will discuss the first embodiment of the present invention. As illustrated in Fig. 1, in a copying machine in accordance with the present embodiment, a transparent document platen 2, on which an original to be copied is placed, is installed on the top surface of the apparatus main body 1, and a document cover 3 for covering the document platen 2 is also installed thereon. Further, an operation section 90, shown in Fig. 2, is installed on the top surface of the apparatus main body 1. The operation section 90 is provided with: a punch key 91 for specifying a punch mode; a display panel 92 for displaying messages; an input key 93; ten keys 94; a clear key 95 for ten keys; a canceling key 96; and a print key 97. The user is allowed to specify various factors, such as various modes, the number of copies, the number of punched holes in the punch mode, and positions of punched holes, through the operation section 90. Moreover, various special modes, such as a thick-paper mode for carrying out a copying operation on sheets of thick paper, a cover mode, and an insert mode, are specified through the operation section 90. As illustrated in Fig. 1, an optical system 4 is installed below the document platen 2. The optical system 4 is constituted of a copy lamp 5 which is a halogen lamp or other lamps, a plurality of mirrors 6 through 11, and a lens unit 12. The mirrors 6 though 11 are arranged so that a light beam projected from the copy lamp 5 is directed to the original placed on the document platen 2, and so that the reflected light beam from the original is directed to a photoreceptor 13, which will be described later, as is indicated by an alternate long and short dash line. Further, the lens unit 12 has a function for refracting and converging the reflected light beam so as to allow it to form a clear image on the photoreceptor 13. Below the optical system, is located a copy process section which has the photoreceptor 13 as its main device. On the periphery of the photoreceptor 13, are disposed a main charger unit 14, a developing unit 15, a transferring charger 16, a separating charger 17, a cleaning unit 18 and other devices. Further, the copy process section also includes a belt-shaped suction unit 19 that is installed on the paper-discharging side of the photoreceptor 13, and a fixing device 20 that is installed on the paper-discharging side of the suction unit 19. Below the developing unit 15, are disposed paired register rollers 21 for supplying sheets of paper to the photoreceptor 13 in proper timing. Further, below the copy process section, are disposed feeding cassettes 22 and 23 for housing sheets of paper of respective sizes. Moreover, a manual feeding tray 24 is attached to the side wall on the developing unit 15 side in the apparatus main body 1. Furthermore, a transporting device 26, which has transporting rollers 25 and other members so as to transport sheets of paper, is installed between the register rollers 21, the feeding cassettes 22, 23 and the manual feeding tray 24. Thus, the register rollers 21, the feeding cassettes 22 and 23, the manual feeding tray 24, and the transporting device 26 constitute a feeding section. On the paper-discharging side of the fixing device 20, are installed a paper-punching device 27, which is related to the present invention and will be described later in detail, and a separation gate 28 for selectively delivering sheets of paper between the feeding cassette 22 and outside of the apparatus main body 1. Further, a staple sorter 29 for ejecting sheets of paper, and other devices are attached to the outer wall on the paper-discharging side of the paper-punching device 27 in the apparatus main body 1. In the copy process section, when the reflected light beam, which has been directed from the original through the optical system 4, forms an image on the photoreceptor 13 that is charged by a predetermined voltage applied from the main charger unit 14, an electrostatic latent image corresponding to the image of the original is formed on the photoreceptor 13. Then, the electrostatic latent image is visualized by toner supplied from the developing unit 15 to form a toner image. This toner image is transferred by the transferring charger 16 onto a sheet of paper that is fed from the feeding cassette 22 or 23 or the manual feeding tray 24. The sheet of paper bearing the toner image transferred thereon is separated from the photoreceptor 13 by the separating charger 17, and is transported by the suction unit 19 to the fixing device 20, where the toner image is fixed onto the sheet of paper by heat. After completion of the copying process as described above, the sheet of paper is subjected to a punching process at the paper-punching device 27, and is directed to the staple sorter 29 through the separation gate 28. Then, the sheet of paper is subjected to a stapling process and a sorting process at the staple sorter 29, and is discharged. Here, in the case of a double-sided copying operation, the sheet of paper, which has passed through the fixing device 20, is directed to the feeding cassette 22 by the separation gate 28, and is subjected to a copying process on the back side. Here, the punching process is carried out by the paper-punching device 27 only when the punch key 91 on the operation section 90 is pressed so that the punch mode is on. The copying machine of the present embodiment is provided with a punching unit 30 as the paper-punching device 27. The following description will discuss the arrangement of the punching unit 30. As illustrated in Fig. 3(a) and Fig. 4, the punching unit 30 is constituted of a transport guide 31, a photosensor 32, a punching device 33, a driving roller 34, a driven roller 35, and a driving device 36. The transport guide 31, which is constituted of two plates, an upper plate 31a and a lower plate 31b, that are disposed at upper and lower positions in parallel with each other, is designed to direct sheets of paper P from the separation gate 28 toward the paper-discharging side, as well as regulating the dislocation of the sheets of paper P in the longitudinal direction. An opening 31c is provided in the upper plate 31a on the downstream side (on the staple sorter 29 side) of the separation gate 28. The photosensor 32 is installed at this opening 31c. The photosensor 32 is a so-called optical sensor of the reflection type. The photosensor 32 projects light downward, and upon receipt of the reflected light from a sheet of paper P being transported through the transport guide 31, it releases a detection signal that indicates the passage of the sheet of paper P. Therefore, the photosensor 32 is used for detecting the rear edge of the sheet of paper P when it stops releasing the detection signal, thereby functioning as a rear-edge detection means. The punching device 33 is installed on the downstream side from the opening 31c in the transport guide 31. The punching device 33 is constituted of a housing case 37, a punching member 38, a resetting spring 39, and a punching die 40. Further, as many punching devices 33 as the number of punch holes required for the sheets of paper P are installed side by side in the direction perpendicular to the paper surface in Fig. 3(a) with predetermined intervals. Fig. 4 is a drawing that is obtained when Fig. 3(a) is seen from the downstream side. Referring to this drawing, the following description will discuss the housing case 37 in detail. The housing case 37 has a space inside such that the punching member 38 is allowed to move up and down and the resetting spring 39 is allowed to extend in a predetermined range. Moreover, the housing case 37 has through holes 37a and 37b provided in its top face and bottom face, which allow the punching member 38 to penetrate therethrough. The through hole 37b coincides with an opening 31d that is provided in the upper plate 31a. The punching member 38, which has a cylindrical shaft shape in a whole view, is provided with a punching blade 38a at its bottom end and a collar portion 38b around its virtually middle portion. A blade 41, shown in Figs. 6(a) through 6(c), or a blade 42, shown in Figs. 7(a) and 7(b), may be adopted as the punching blade 38a. The blade 41, which is commonly used in manual punching devices, has two blade tops 41a with a deep recessed portion 41b between the blade tops 41a. In this case, however, when the blade 41 is used to form a punch hole in a sheet of paper P, only the blade tops 41a stick into the sheet of paper P at its initial stage of the punching operation. When the transporting force is applied to the sheet of paper P at this state, the sheet of paper P tends to be torn easily. Therefore, the blade 41 is not suitable for a high-speed punching operation with the sheet of paper P being transported. The blade 42, on the other hand, has a recess like a mortar on its top, and all the circumferential edge on the top forms a blade top 42a. Therefore, even if the blade 42 is used to form a punch hole in a sheet of paper P, no trouble occurs since the blade top 42a stick into the sheet of paper at the same time, which is different from the case of the blade 41. In the blade 42, however, since all the portion of the blade top 42a has to stick into the sheet of paper P at the same time, it is necessary to provide an extremely large force as a driving torque for the punching member 38; this results in a heavy burden on the driving device 36. Therefore, it is difficult to adopt the blade 42 in an actual operation. For this reason, a blade 43, shown in Figs. 8(a) through 8(d), is adopted in the present punching unit 30. The blade 43 has four blade tops 43a, provided around its center axis, and four blade bottoms 43b that are formed between the adjacent blade tops 43a. Here, the blade tops 43a and the blade bottoms 43b are located alternately with intervals of 45 degrees. The blade 43 is designed so that the minimum stroke required for punching a hole is set to be smaller (0.8 mm) than those of the blades 41 and 42. The blade tops 43a are formed into a cruciform shape; therefore, upon punching a hole in a sheet of paper P, the blade tops 43a stick into the sheet at the same time, and it is possible to minimize the driving torque of the punching member 38, compared with the case of the blade 42. Thus, this arrangement is suitable for carrying out a punching operation on a sheet of paper P at high speeds while the sheet of paper P is being transported. The punching die 40 is attached to the lower plate 31b. The punching die 40 has a tube section 40a at its central portion, which extends upward and has an opening at its top. The tube section 40a, which has a diameter slightly larger than the outer diameter of the punching member 38, is inserted into an opening 31e that is provided in the lower plate 31b so as to face the opening 31d. With this arrangement, when the punching member 38 is shifted downward, the punching blade 38a reaches the inside of the tube section 40a. The resetting spring 39, which is a compression coil spring, is disposed around the punching member 38. The respective ends of the resetting spring 39 are held by the collar portion 38b and the bottom surface of the housing case 37. When no external downward force is applied onto the punching member 38, the resetting spring 39 urges the punching member 38 to a stand-by station, as illustrated in Fig. 3(a). Further, when an external downward force, which has been applied to the punching member 38, is released, the resetting spring 39 resents the punching member 38 to the stand-by station. The driving roller 34 and the driven roller 35 are installed on the downstream side (on the staple sorter 29 side) from the punching device 33 in the transport guide 31. The driving roller 34 (hereinafter, referred to simply as the transport roller) is installed at an opening 31f that is provided in the lower plate 31b, and is driven to rotate by a motor, not shown. The driven roller 35 (hereinafter, referred to simply as the transport roller), on the other hand, is installed at an opening 31g that is provided in the upper plate 31a, and is driven to rotate by contacting the transport roller 34. The transport rollers 34 and 35 always rotate in a direction indicated by the arrow during the copying operation, thereby transporting sheets of paper P toward the downstream side. When the transporting operation is carried out in a state where a large friction is exerted between the transport rollers 34 and 35 and the sheet of paper P, the sheet of paper P might be damaged depending on the kinds of the sheet of paper P. In other words, the sheet of paper P is pulled by the transport rollers 34 and 35 even while it is caught by the punching blade 38a; therefore, in the case of thin paper, the portion of the sheet of paper P that is caught by the punching blade 38a tends to be torn. For this reason, in the present embodiment, a foamed material is adopted as a material of the transport roller 35 so that the transport roller 35 may slip slightly with respect to the sheet of paper P in the case when a force opposing to the transporting force (a restraint due to the punching blade 38a) is exerted. Materials having sponge texture may be preferably used as such a foamed material; however, those foamed materials to be used as the transport roller 35 should satisfy rigid requirements in terms of physical properties, weatherability, and other properties, in comparison with commonly-used foamed materials. The materials that satisfy those requirements include, for example, urethane foam and silicon-rubber foam. The transport roller 35, which is made of a foamed material having such properties, presses the sheet of paper P at a virtually constant force because of its foamed structure, independent of its amount of deformation; therefore, it is possible to obtain such a slipping performance. In contrast, rubber rollers, which has a greater pressing force when deformed to a great extent, are not suitable for the transport roller 35. Moreover, even in commonly-used arrangements wherein a rubber roller is used as the transport roller 34 and the transport roller 35 is made of POM (Polyoxymethylene), the transport roller 35 may be also allowed to slip with respect to the sheet of paper P by setting the nipping force of the transport rollers 34 and 35 smaller than usually used. When comparisons are made between the transport rollers 35 that are respectively made of urethane foam and POM, there is hardly any difference between their slipping performance as long as the nipping force of the transport rollers 34 and 35 is appropriately set. However, in the case of POM rollers, since they have virtually rigid bodies, deviations might occur in their slipping performance unless the nipping force of the transport rollers 34 and 35 are strictly set. In other words, if the nipping force is too large, the punching operation will not be properly performed due to insufficient slipping. If the nipping force is too small, slipping might occur even during the transporting process. In contrast, in the case of urethane-foam rollers, the pressing force to the sheet of paper P is kept virtually constant independent of the amount of deformation; therefore, optimum slipping performance is obtained without the necessity of setting the nipping force as strictly as that of POM rollers. Additionally, the difference in slipping performance between POM rollers and urethane-foam rollers will be clarified by the results of experiments which will be described later. The driving device 36 is constituted of a support member 51, a pressing bar 52, a piezoelectric element 53, and a distortion-enlarging mechanism 54. The support member 51, which is installed on the paper guide 31, consists of a base 51a and a pillar 51b. The base 51a is securely fixed on the upper plate 31a, and the pillar 51b extends upward vertically from the base 51a. The pressing bar 52 has its base end attached to the top end of the pillar 51b with a pin 55 so as to rotate freely, and has its free end extending to the top end of the punching member 38. The piezoelectric element 53 exerts a dimensional distortion through its piezoelectric effects when voltage is applied thereto. As shown in Fig. 5, the directions of the dimensional distortion include a direction extending toward the center with respect to the longitudinal direction of Fig. 5 and a direction extending outward with respect to the lateral direction of Fig. 5. The distortion-enlarging mechanism 54, which is made of steel having a thickness in the order of 5 mm in its entire structure, is partially provided with portions that are easily distorted (portions encircled with an alternate long and short dash line) so that the entire structure is distorted by the dimensional distortion of the piezoelectric element 53. The distortion-enlarging mechanism 54 is constituted of side portions 54a and 54b, an upper portion 54c, a lower portion 54d, and connecting portions 54e and 54f, all of which surround the piezoelectric element 53. The side portions 54a and 54b are connected to the respective ends of the piezoelectric element 53 in the lateral direction of Fig. 5. The upper portion 54c and the lower portion 54d are respectively connected to the side portions 54a and 54b through the narrowed portions that are located at the respective ends thereof, and each of them has a large notched portion at the center thereof so as to be easily distorted. With this structure, the distortion-enlarging mechanism 54 has distortions at portions indicated by alternate long and short dash lines in Fig. 5. Here, since the amounts of the distortions are slight at the distorted portions, no plastic distortion occurs. The connecting portion 54e is formed into a long shape extending upward from the top end at the center of the upper portion 54c, and its upper end is fixed to a shaft in the pressing bar 52 at a position relatively closer to the base and farther from the free end. The connecting portion 54f, on the other hand, is formed into a short shape extending downward from the bottom end at the center of the lower portion 54d, and its lower end is fixed to a shaft in the base 51a. In the driving device 36 that is arranged as described above, the dimensional distortion, which is exerted on the piezoelectric element 53 in the directions indicated by the arrows, is increased to a larger displacement by the distortion of the distortion-enlarging mechanism 54, and the displacement is transmitted to the pressing bar 52. Then, the pressing bar 52 is pulled toward the base 51a side so that it rotates downward centered on the pin 55. Thus, the free end of the pressing bar 52 is shifted downward, and presses the punching member 38 downward. The following description will discuss an outline of a control system for driving the punching device 33. As shown in Fig. 9, in this control system, a detection signal from the photosensor 32 is inputted to a timer 61. The timer 61, upon receipt of the detection signal, starts time-counting, and after counting a predetermined period of time, releases a time-counting completion signal to a driving circuit 62. The driving circuit 62 is a circuit for generating a driving voltage to be supplied to the piezoelectric element 53, and upon receipt of the time-counting completion signal from the timer 61, the driving circuit 62 releases the driving voltage. The time that is counted by the timer 61 is determined based on the transporting speed, punch-hole positions on a sheet of paper P, the operating time of the punching device 33 and the driving device 36, and other factors. For example, supposing that the transporting speed is constant, the counting time of the timer 61 is set longer in the case of forming a hole close to the rear edge of the sheet of paper P, while the counting time of the timer 61 is set shorter in the case of forming a hole far from the rear edge of the sheet of paper P. Here, supposing that the transporting speed is V [mm/sec] and the time during which the sheet of paper P is caught by the punching blade 38a is t [sec], the punching member 38 is driven under conditions where the following inequality holds so that the sheet of paper P is not damaged by the punching operation. V [mm/sec] × t [sec] ≤ 1 [mm] More specifically, the present punching unit 30 is arranged so that the transport roller 35 is allowed to make a slip of 1 [mm] at maximum with respect to the sheet of paper P when the sheet of paper P is caught by the punching blade 38a. Supposing that the transporting speed is set as fast as that of commonly-used copying machines, the above-mentioned conditions are satisfied by using the piezoelectric element 53 so as to shorten the operating time of the driving device 36. In addition, the above-mentioned relationship will be further clarified by the results of experiments, which will be described later. Referring to the flow chart of Fig. 10, the following description will discuss the operation of the punching unit 30. A sheet of paper P, which has been transported from the main body 1, is directed into the transport guide 31, and then transported by the transport rollers 34 and 35. In this case, when the rear edge of the sheet of paper P is detected by the photosensor 32 (S1), a judgement is made as to whether or not the punch mode has been specified (S2). If the punch mode has been specified, the timer 61 turns on, thereby starting time-counting (S3). The timer 61, after counting a predetermined period of time, turns off, thereby completing the time-counting (S4). Upon receipt of the OFF of the timer 61, the driving device 36 and the punching device 33 are activated, and a punching operation is carried out (S5). Here, if the punch mode is not specified at S2, the punching operation is not carried out. During the punching operation, the driving circuit 62, upon receipt of the time-counting completion signal from the timer 61, generates a driving voltage for driving the piezoelectric element 53. Thus, in the driving device 36, a dimensional distortion occurs on the piezoelectric element 53, and the pressing bar 52 is driven downward. In the punching device 33, since the punching member 38 is depressed downward by the pressing bar 52, the punching blade 38a catches the sheet of paper P, and forms a punch hole, as illustrated in Fig. 3(b). At this time, since the transport rollers 34 and 35 are rotating, the transport roller 35 is allowed to slip with respect to the sheet of paper P when the sheet of paper P is caught by the punching blade 38a. The following description will discuss the experimental results on actual punching operations that were carried out by the punching unit 30 while one portion or all portions of the sheet of paper was being transported without stop. Here, explanations will be first given on comparative examples (A) wherein both the transport rollers 34 and 35 are made of rubber, and then given on examples (B) wherein the transport roller 34 is a rubber roller and the transport roller 35 is a polyurethane-foam roller. In this case, the transporting speed V [mm/sec] at which the sheet of paper P is transported was classified into: 100, 200, and 400; and the time t [sec] during which the sheet of paper is caught by the punching blade 38a was classified into 1.25/1000, 2.5/1000, 5/1000, 10/1000, 20/1000 and 40/1000. The experiments were carried out under various combinations of the transporting speed V and the time t. The time t represents a period of time from the time when the punching blade 38a sticks into the sheet of paper P until the time it retreats from the sheet of paper P after completion of the punching process. Further, the frequency of occurrence of troubles, such as torn sheets of paper P and scratches on paper P due to slip of the transport roller 35, was classified by each scale of 10%, and based on the accumulations of the frequencies of occurrence, histograms were made on the respective cases of (A) and (B), as shown Figs. 11(a) through 11(c) and Figs. 12(a) through 12(c) respectively. Additionally, five types of paper P were used in the present experiments: 64 g/m 2 , 75 g/m 2 , 80 g/m 2 , 128 g/m 2 and 200 g/m 2 . Moreover, in the above-mentioned histograms, the count value is determined by one value of "V" and one value of "t". In addition, the types of tested sheets of paper are related to the values of "V" and "t", in such a manner that, for example, in thick sheets of paper, as the transporting speed V decreases, the time t increases. Furthermore, the numbers (parameter) of the five types of tested sheets of paper were not uniform, and phenomena of troubles caused on the various tested sheets and their levels were different depending on the respective cases. The histograms in the case of (A) are shown in Figs. 11(a) through 11(c). In Fig. 11(b), the numbers of sheets of tested paper that were used under the conditions of V = 200 [mm/sec] and t = 40/1000 [sec] were: 30 sheets of 64 g/m 2 ; 40 sheets of 75 g/m 2 ; 40 sheets of 80 g/m 2 ; 50 sheets of 128 g/m 2 ; and 40 sheets of 200 g/m 2 . During punching operations under these conditions, the following troubles occurred on the respective sheets of tested paper. As shown in Fig. 13(a), tears R that extend to the rear edge of the sheet developed in the respective sheets of tested paper: 64, 75, and 80 g/m 2 . As shown in Fig. 13(b), defective punch holes S were caused in the respective sheets of tested paper: 64, 75, 80 and 128 g/m 2 . As shown in Fig. 13(c), scratches T due to the roller were caused in the respective sheets of tested paper: 75, 128, and 200 g/m 2 . Here, the above-mentioned phenomena of troubles are regarded as troubles that are caused by application of the transporting force to a sheet of paper that is being caught by the punching blade 38a. Thus, the rate of occurrence of troubles is calculated as follows: (The total number of occurrences of troubles) / (The total number of all the sheets of tested paper) × 100 [%] When the experimental results, which are indicated by the histograms of Figs. 11(a) through 11(c), are evaluated and analyzed systematically, it is found that the frequency of occurrence of troubles is not more than 10 % in the case when the aforementioned inequality (1) is satisfied. In other words, these cases correspond to t = 1.25, 2.5, 5, and 10 [sec] in the histogram (V = 100 [mm/sec]) of Fig. 11(a); t = 1.25, 2.5, and 5 [sec] in the histogram (V = 200 [mm/sec]) of Fig. 11(b); and t = 1.25 and 2.5 [sec] in the histogram (V = 400 [mm/sec]) of Fig. 11(c). This shows that if the amount of transport of the sheet of paper P, which is made by the transport roller 35 while the sheet of paper P is being caught by the punching blade 38a, is not more than 1 [mm], damages caused on the sheet of paper P are comparatively small. Therefore, if the punching unit 30 is designed so as to provide the conditions that satisfy the inequality (1), it becomes possible to carry out the punching operation while the sheet of paper P is being transported. However, even under the conditions that satisfied the inequality (1), troubles occurred although the percentage was not more than 10 %. When consideration was given to clarify the causes of this problem, it was found that a major cause was that the force used to depress the sheet of paper P was too strong because both of the transport rollers 34 and 35 were made of rubber. Here, other experiments were carried out so as to find whether or not the troubles could be solved by reducing the nipping force of the transport rollers 34 and 35. However, even if the average pressing force was reduced, it was not possible to eliminate the troubles completely although the rate of occurrence of troubles was lowered. In other words, as long as the transport rollers 34 and 35 are made of rubber, it seems impossible to completely eliminate the phenomenon that the sheet of paper P closely contact the transport rollers 34 and 35 momentarily. Next, with regards to the case (B), experiments were carried out so as to check the occurrence of troubles during the punching operation in the same manner as was done in the case (A). More specifically, in the experiments, rubber was used as the material of the transport roller 34, and foamed material, such as polyurethane foam, was used as the material of the transport roller 35. The same experiments were also carried out in the case of using POM resin as the material of the transport roller 35. The experiments showed that in both the foamed material and the POM resin, the rate of occurrence of troubles was lowered to far less than 10 %, and the frequency of occurrence of troubles was further lowered even in the case of V × t > 1 . This is because slipping, which is allowed by the transport rollers 34 and 35, absorbs the transporting force exerted on the sheet of paper. In other words, the sheet of paper P is being pulled by the transport rollers 34 and 35 even while it is caught by the punching blade 38a. Therefore, it is pulled harder beyond its slight flexibility, but the pulling force at this time is absorbed by the slipping of the sheet of paper allowed by the transport rollers 34 and 35. Among the above-mentioned upgrading experiments, the results of those using a rubber roller and a polyurethane-foam roller respectively as the transport roller 34 and the transport roller 35 are shown in histograms in Figs. 12(a) through 12(c). These histograms show that troubles are completely eliminated under conditions that satisfy V × t ≤ 1 and that the frequency of occurrence of troubles is lowered even under conditions where V × t > 1 . The following points were found from the results of the experiments in the respective cases (A) and (B). In the case when the time t needed for catching the paper was extremely short in comparison with the transporting speed V, no troubles occurred in any of the sheets of paper. In the case when the time t needed for catching the paper was extremely long in comparison with the transporting speed V, troubles, such as tears R (see Fig. 13(a)) and defective punch holes S (see Fig. 13(b)), occurred in thin paper at high frequencies. Moreover, as to thick paper, under the same conditions, since the sheet of paper P came into a stopped state momentarily, scratches T (see Fig. 13(c)) due to slip of the transport rollers 34 and 35 were caused only in the case (A) at high frequencies. As described above, in the punching unit 30 of the present embodiment, since the punching operation is carried out on the rear side of the sheet of paper P, it is possible to reduce the frequency of occurrence of troubles such as paper jam to a great degree, even if the transporting speed is increased in order to achieve a high-speed operation. Further, since warping of the sheet of paper P does not occur, it is possible to avoid damages due to the warping of the sheet of paper P, as well as allowing the punching operation to be conducted on thick paper that exceeds 1.28 g/m 2 . Moreover, the punching operation is carried out without the necessity of stopping the transportation while the transport rollers 34 and 35 are kept rotating; this enables a high-speed operation. Furthermore, the transport roller 35 allows a slight slip during the short period when the sheet of paper P is caught by the punching blade 38a upon forming punch holes; this reduces the possibility of damages such as torn paper even if the sheet of paper P is pulled by the transport rollers 34 and 35. In addition, since the piezoelectric element 53 is adopted as a driving source of the driving device 36, the operation speed of the punching device 33 is increased; the time during which the sheet of paper P is caught by the punching blade 38a is shortened; and thus it becomes possible to reduce the frequency of occurrence of damage to the sheet of paper P. Furthermore, when a stapling operation is carried out simultaneously with the punching operation, this arrangement allows both of the operations to be carried out on the rear-edge side of the sheets of paper; thereby improving the efficiency of the operations. Commonly, the stapling operation is conducted after aligning the rear edges of sheets of paper. Therefore, sheets of paper P with punch holes on the rear edges thereof have less misalignments between the punch holes compared with other cases. In addition, in the stapling operation, it is common to staple the sheets of paper on their rear-edge side, with the alignment of punch holes coincident with the direction of the stapling operation. [MODIFIED EXAMPLE 1] The following description will discuss the first modified example of the present embodiment. In this modified example, a punching unit 71, shown in Fig. 14, is provided as the paper-punching device 27. The punching unit 71, which has punching devices 33 and 33', and a driving device 72, is designed to form two punch holes. The punching devices 33 and 33', which have identical functions, are disposed with a predetermined interval that corresponds to the interval of punch holes. A driving device 72, which functions as a driving means, is disposed at the mid-position between the punching devices 33 and 33'. The driving device 72 is constituted of a support member 73, pressing bars 74 and 75, a piezoelectric element 53, and a distortion-enlarging mechanism 54. The support member 73, which is installed on the paper guide 31, consists of a base 73a and pillars 73b and 73c. The base 73a is securely fixed on the upper plate 31a, and the pillars 73b and 73c extend upward vertically from the base 73a in parallel with each other with a predetermined interval. The pressing bar 74 has its base end attached to the top end of the pillar 73b with a pin 76 so as to rotate freely. The free end of the pressing bar 74 extends to the top end of the punching member 38 of the punching device 33. The pressing bar 75, on the other hand, has its base end attached to the top end of the pillar 73c with a pin 77 so as to rotate freely in a direction reversed to the pressing bar 74. The free end of the pressing bar 75 extends to the top end of the punching member 38 of the punching device 33'. The distortion-enlarging mechanism 54 has its connecting section 54e attached to both the pressing bars 74 and 75 with a pin at the mid-position between the pillars 73b and 73c. Further, the connecting section 54f is attached to the base 73a with a pin. In the driving device 72 that is arranged as described above, the dimensional distortion, which is exerted on the piezoelectric element 53, is increased to a larger displacement by the distortion of the distortion-enlarging mechanism 54, and the displacement is transmitted to the pressing bars 74 and 75. Then, since their respective attached portions to the connecting section 54e are pulled toward the base 73a side, the pressing bars 74 and 75 rotate downward centered on the pins 76 and 77. Thus, the free ends of the pressing bars 74 and 75 are shifted downward, and press the punching members 38 of the punching devices 33 and 33' downward. In this modified example, the two punching devices 33 and 33' are driven by a single driving source using the piezoelectric element 53; this makes it possible to simplify the construction, as well as reducing the manufacturing cost of the punching unit 71. [MODIFIED EXAMPLE 2] The following description will discuss the second modified example of the present embodiment. In this modified example, a punching unit 81, shown in Fig. 15, is provided as the paper-punching device 27. The punching unit 81 is provided with three punching devices 33 and a driving device 82 (driving means) for driving these punching devices 33, and the punching devices 33 are securely fixed on the upper plate 31a of the transport guide 31. The punching devices 33 are disposed so that their punching members 38 are aligned on a straight line that makes an angle (90 - )° with respect to the transporting direction, and so that the intervals between the adjacent punching members 38 and 38 (that is, the punching blades 38a and 38b) in the direction orthogonal to the transporting direction are set to a constant value x [mm]. Further, the driving device 82 drives the respective punching devices 33 individually by using three driving devices 36, not shown. Here, the punching devices 33 are driven by a single driving circuit 62, and are not driven at the same time. Therefore, they are driven in a sequential manner from the punching device 33(α) through the punching device 33(β) to the punching device 33(γ) with predetermined time-intervals. In the punching unit 81 that is arranged as described above, a sheet of paper P, which is being transported, is first subjected to a punching operation by the punching device 33(α), next subjected to a punching operation by the punching device 33(β), and then subjected to a punching operation by the punching device 33(γ). Thus, the sheet of paper P has punch holes H that are aligned in a straight line as shown in Fig. 16. Here, supposing that the adjacent punching devices 33 are successively driven with a time-interval of T [sec] at a transporting speed of V [mm/sec], the relationship indicated by the following equation has to be satisfied in order that the punch holes H, formed by the punching devices 33, are aligned in a straight line along the rear side on the sheet of paper P under these conditions. Therefore, the driving circuit 62 is arranged to drive the punching devices 33 based on the following relationship. tan = V·T/x More specifically, supposing that T = 50/1000 [sec], V = 300 [mm/sec], and x = 108 [mm] (corresponding to the U.S. specification),  = 7.91° is obtained from the equation (2) (that is, from the equation: tan = (300 × 50/1000)/108 = 0.139) . When the punching operation is carried out using this setting, three punch holes H corresponding to the U.S. specification are properly formed. With this modified example, although it is necessary to install as many punching devices 33 and driving devices 36 as the number of the punch holes H, it is only necessary to provide one driving circuit 62. This makes it possible to simplify the construction of the control system, as well as reducing the manufacturing cost of the punching unit 81. [EMBODIMENT 2] Referring to Fig. 1 and Figs. 17 through 20, the following description will discuss the second embodiment of the present invention. Here, those members that have the same functions and that are described in the first embodiment are indicated by the same reference numerals and the description thereof is omitted. The copying machine of the present embodiment is provided with a punching unit 101, shown in Fig. 17, that is installed in the main body 1 shown in Fig. 1 and that functions as the paper-punching device 27. This punching unit 101 is designed so that both the punching operation for sheets of extremely thick paper P and the high-speed punching operation for sheets of normal paper P are compatibly carried out. In the punching unit 101, the space between the punching device 33 on the transport guide 31 and the transport rollers 34 and 35 is widened in the punching unit 30 (see Fig. 3(a)), and a punching device 102 is installed in place of the punching device 33. The punching device 102 is a high-speed-use punching device for forming punch holes in sheets of paper ranging from normal paper to quite thick paper. The punching device 102 is constituted of a housing case 103, a punching member 104, a resetting spring 105, and a punching die 106. The punching member 104 is provided with a punching blade 104a at its lower end. Further, although it has the virtually same functions as the punching device 33, the punching device 102 is capable of forming punch holes in sheets of thick paper beyond the order of 200 g/m 2 ; this makes it quite different from the punching device 33. Therefore, in the punching device 102, the urging force of the resetting spring 105 is set to be greater than that of the resetting spring 39, and the cutting performance of the punching blade 104a is set to be higher than that of the punching blade 38a, if necessary. In addition to the aforementioned arrangement, the punching device 102 is provided with an eccentric cam 107. The eccentric cam 107, which has a disc shape, is driven by a motor 113 (see Fig. 18), centered on a rotation axis 107a that is located at an eccentric position. The motor 113 will be described later. Here, the eccentric cam 107 is arranged so that the circumferential edge that is closest to the rotation axis 107a stays in contact with the top of the punching member 104 that is urged to the stand-by station by the resetting spring 105. The eccentric cam 107 makes the eccentric rotation with its circumferential edge always contacting the top of the punching member 104, thereby allowing the punching member 104 to move up and down. Additionally, both the punching devices 33 and 102 have the same positions of punch holes on the sheet of paper P. The following description will discuss an outline of a control system for driving the punching devices 33 and 102. As shown in Fig. 18, in this control system, a detection signal from the photosensor 32 is inputted to a timer 108. The timer 108, upon receipt of the detection signal, starts time-counting, and after counting a predetermined period of time, releases a time-counting completion signal to a driving circuit 112. The timer 108 provides different time-counting periods depending on the operations of the punching device 33 and the punching device 102. The first time-counting period used for operating the punching device 102 is set to be longer than the second time-counting period used for operating the punching device 33. This is because the station of the punching device 102 is farther from the photosensor 32, compared with the punching device 33. Here, a CPU 109 makes a selection as to which time-counting period is used in the timer 108. The CPU 109 instructs the timer 108 to time-count for the first time-counting period when the thick-paper mode is specified, when paper is fed from the manual feeding tray 24, or when the cover mode, or the insert mode is specified. When the thick-paper mode is not on, the CPU 109 instructs the timer 108 to time-count for the second time-counting period. The above-mentioned modes are specified by the user through the operation section 90, and the selected modes are stored in a RAM 111 as mode information. The mode information is called for by the CPU 109, if necessary. A driving circuit 112 is a circuit for driving the piezoelectric element 53 and the motor 113. In other words, the driving circuit 112, upon receipt of the time-counting completion signal for the first time-counting period from the timer 108, releases a voltage to the motor 113. Further, the driving circuit 112, upon receipt of the time-counting completion signal for the second time-counting period from the timer 108, releases a voltage to the piezoelectric element 53. Moreover, in the present punching unit 101, when the thick-paper mode is on, the CPU 109 temporarily stops the rotation of the transport rollers 34 and 35 while the punching device 102 is operated. In contrast, when the thick-paper mode is not on, the punching operation is carried out with the transport rollers 34 and 35 rotating, in the same manner as the first embodiment. In the punching unit 101 that has the above-mentioned arrangement, the photosensor 32 detects the rear edge of sheets of paper P. In the case of using thick paper as the paper P, if the thick-paper mode has been specified by the user through the operation section 90 prior to the copying operation, the driving circuit 112 supplies the voltage to the motor 113 after the timer 108 has time-counted for a predetermined period. Thus, the motor 113 rotates, allowing the punching device 102 to be driven. At this time, since the paired transport rollers 34 and 35 are stopped, punch holes are formed while the sheet of paper P is stopped. During the punching operation, the eccentric cam 107 is driven by the motor 113 to make a 180°-rotation, and the resulting force causes the punching member 38 to move down and let the punching blade 38a to stick through the sheet of paper P. When the eccentric cam 107 makes another 180°-rotation, the force from the eccentric cam 107 is released, and the punching member 104 is urged upward by the resetting spring 105, thereby completing the punching operation. Thereafter, the transport rollers 34 and 35 are rotated again, and the sheet of paper P is discharged. Referring to the flow chart of Fig. 19, the following description will discuss the operation of the copying machine of the present embodiment. First, a judgement is made as to whether or not the punch mode has been specified (S11). If the punch mode has been specified, a judgement is made as to whether or not the thick-paper mode has been specified (S12). If the thick-paper mode is not on, a judgement is further made as to whether or not the feeding from the manual feeding tray has been specified (S13). If the feeding from the manual feeding tray is not specified, a judgement is successively made as to whether or not the cover mode has been specified (S14). Since the kind of sheets of paper P to be set on the manual paper tray 24 is not clearly identified, the step S14 is prepared, assuming that thick paper is set thereon. If the cover mode is not specified, a judgement is made as to whether or not the insert mode is specified (S15). If the cover mode or the insert mode has been specified, a judgement is made as to whether or not it is possible to feed sheets of paper from either the feeding cassette 22 or 34 wherein the cover-use paper P or the insert-use paper P is provided (S16). If the cover-use paper P or the insert-use paper P is available, the copying operation is carried out (S17). After completion of the copying operation, the punching operation of the temporarily stopping type for thick paper is carried out by the punching device 102 (S18), and when the sheet of paper P is discharged (S19), all the operations are completed. In contrast, if the punch mode is not on at S11, the copying operation is carried out, as it is (S20), and the sequence proceeds to S19. Here, if the thick-paper mode is selected at S12, and if the feeding is made from the manual feeding tray 24 at S13, the sequence proceeds to S17. Further, if the insert mode is not on at S15, or if the cover-use paper P or the insert-use paper P is not fed at S16, the copying operation is carried out (S21). After completion of the copying operation, the punching operation of the high-speed type for normal paper is carried out (S22), and the sequence proceeds to S19. As described above, in the punching unit 101 of the present embodiment, the piezoelectric element is adopted as the power source for the punching device 33 for normal-paper use, and the motor 113 is adopted as the power source for the punching device 102 for thick-paper use. This arrangement, which uses the punching devices 33 and 102 separately depending on the thickness of the sheet of paper P, makes it possible to form punch holes through sheets of thick paper used in the insert mode, cover mode and other modes that weighs not less than 200 g/m 2 . Moreover, when the punching operation is carried out on sheets of normal paper, the punching device 33 provides a high-speed punching operation and makes it possible to lower the frequency of occurrence of damage to the sheets of paper P, in the same manner as described in the first embodiment. [MODIFIED EXAMPLE] In the copying machine of the present modified example, a paper-stand-by section 121, shown in Fig. 20, is provided inside the main body shown in Fig. 1. The paper-stand-by section 121, which is located between the register rollers 21 and the transport rollers 25, is constituted of a lower plate 122, upper plates 123 and 124, and a pressure sensor 125. The lower plate 122 is disposed on the lower side of the transport path of sheets of paper P so as to guide the sheets of paper P. The upper plate 123 is disposed in a tilted manner so that its one end is located in the vicinity of the mid-point between the register rollers 21 while the other end is located at a position slightly higher than the former end. The upper plate 124, on the other hand, is disposed in a tilted manner so that its one end is located in the vicinity of the mid-point between the transport rollers 25 while the other end is located at a position slightly higher than the former end. In other words, the upper plates 123 and 124 form guiding plates that are raised upward at the mid-point between the register rollers 21 and the transport rollers 25. The pressure sensor 125 is installed in the space between the upper plate 123 and the upper plate 124. The pressure sensor 125 is a semiconductor element (piezo element) which is capable of making an analog-type detection of pressure (force) that is applied upon the surface thereof. As to the pressure sensor 125, for example, the semiconductor pressure transducer P-8100 manufactured by Copal Electronics Corp. is preferably used. The pressure sensor 125, which is disposed at the position as described above, is thus arranged so that, when a sheet of paper P, transported by the transport rollers 25, is blocked by the register rollers 21 and is warped upward, it detects the pressing force of the warped portion of the sheet of paper P. In other words, the pressure sensor 125 detects the pressing force as the stiffness of the sheet of paper P. In the present copying machine, the CPU 109 makes a judgement as to whether the sheet of paper P in question is thick paper or normal paper in accordance with the detection output from the pressure sensor 125, and supplies the result of the judgement to the timer 108. In other words, if the sheet of paper P is thick paper, the CPU 109 instructs the timer 108 to time-count for the first time-counting period. If the sheet of paper 108 is normal paper, the CPU 109 instructs the timer 108 to time-count for the second time-counting period. That is, in the present modified example, the result of the judgement, which is made as to whether or not the sheet of paper is thick paper, is utilized in place of the thick-paper mode that has to be specified by the user. In the copying machine having the arrangement as described above, prior to the transferring process of a toner image formed on the photoreceptor 13, the sheet of paper P is blocked by the register rollers 21 at its leading edge, and since it is still transported by the transport rollers 25 by a predetermined amount, the sheet of paper P is stopped with a warp having a predetermined size. This arrangement makes it possible to eliminate any skew in the sheet of paper at its leading edge, thereby providing a proper orientation of the paper. At this time, the pressure sensor 125, which is pressed by the warped paper P, detects the pressure, thereby releasing a detection signal. In accordance with the detection signal, a judgement is made as to whether or not the sheet of paper is thick paper or normal paper. Thereafter, the register rollers 21 rotate in synchronism with the optical system 4, shown in Fig. 1, and the sheet of paper P is supplied to the photoreceptor 13 through the transport guide 126. The sheet of paper P, upon completion of the copying operation after having been subjected to the predetermined processes such as transferring process, is transported to the punching unit 101, shown in Fig. 17. The sheet of paper P, if judged as thick paper, is subjected to the punching operation in the punching device 102, and if judged as normal paper, is subjected to the punching operation in the punching device 33. As described above, in the present modified example, the punching devices 33 and 102 are separately used depending on the judgements that are made by utilizing the detection output of the pressure sensor 125 as to whether or not the sheet of paper in question is thick paper. Therefore, it is not necessary for the user to set the thick-paper mode. [EMBODIMENT 3] Referring to Fig. 1 and Figs. 21 through 24, the following description will discuss the third embodiment of the present invention. Here, those members that have the same functions and that are described in the first and second embodiments are indicated by the same reference numerals and the description thereof is omitted. In addition to the copying machine having the arrangement described in the first embodiment, the copying machine of the present embodiment, which has the arrangement shown in Fig. 1, is further provided with a controlling function for improving the reliability of the punching operation. Moreover, the present copying machine has a punching unit 151, shown in Fig. 21, as the punching device 27. Although the punching unit 151 has virtually the same functions as the punching device 33 (see Fig. 3) of the first embodiment, it is provided with a punching device 152 in place of the punching device 33. The punching device 152 is constituted of a housing case 153, a punching member 38, a resetting spring 39, a punching die 40, and a photosensor 154. The housing case 153 is provided with a passage hole 153a that penetrates from the upper side of the upper plate 31a to the inside of the transport guide 31. The passage hole 153a is a through hole that penetrates from a connecting portion between the outer wall of the housing case 153 and the flat portion that is fixed to the upper plate 31a to the vicinity of a passage aperture 153b. The central axis of the passage hole 153a passes through the tube section 40a of the punching die 40. The photosensor 154 is an optical sensor of the transmission type having an light-emitting section 154a and a light-receiving section 154b. The light-emitting section 154a is located at the proximity of the opening on the upper side of the passage hole 153a. The light-receiving section 154b, on the other hand, is disposed so as to face the light-emitting section 154a through the passage hole 153a and the tube section 40a. With this arrangement, the light-receiving section 154b receives light emitted by the light-emitting section 154a when there is no paper P in the proximity of a passage aperture 153b inside the transport guide 31. Thus, the photosensor 154 functions as a paper-detection means. As shown in Fig. 22, in a control system for driving the punching device 152, the detection signal from the photosensor 154 is supplied to a CPU 155 for controlling the operations of the present copying machine. The CPU 155, upon functioning as an idle-driving control means, makes a judgement as to the presence or absence of the sheet of paper P in accordance with the detection signal. If the judgement shows that there is no paper P inside the transport guide 31, the CPU 155 controls the driving circuit 62 so as to drive the piezoelectric element 53. In contrast, if the judgement shows that there is paper P inside the transport guide 31, the CPU 155 sends a message for warning the occurrence of a paper jam to the display panel 92 on the operation section 90. Referring to the flow chart of Fig. 23, the following description will discuss the operation of the copying machine that is provided with the punching unit 151 having the above-mentioned arrangement. When the power switch, not shown, on the operation section of the main body 1 is turned on (S21), the warm-up process is first carried out in the main body 1, prior to the copying operation (S22). During the warm-up process, the photosensor 154 carries out a detection to find any paper jam inside the punching unit 151. (S23). If a paper jam occurs, the sheet of paper P blocks light emitted from the light-emitting section 154a such that the light-receiving section 154b is not allowed to receive the light and to release the light-receipt signal. In contrast, if the sheet of paper is transported normally after the punching operation, the light from the light-emitting section 154a is received by the light-receiving section 154b, thereby allowing the light-receiving section to release the light-receipt signal. Next, the CPU 155 confirms the occurrence of paper jam in accordance with the output from the photosensor 154 (S24). If there is no paper jam, the punching member 38 is driven one time to execute an up-and-down movement with no sheet of paper P (S25). Prior to this process, the CPU has supplied a voltage to the piezoelectric element 53 during the warm-up process, and the piezoelectric element 53 thus makes an electrical discharge at S25, thereby allowing the punching member 38 to execute the up-and-down movement once. If the piezoelectric element 53 is left with static electricity accumulated therein, that is, if the electrical discharge is not made, the punching member 38 will be kept at the lowered state and block the sheet of paper P inside the transport guide 31. In contrast, this arrangement, which allows the punching device 38 to make the up-and-down movement at S25, makes it possible to return the punching member 38 to the stand-by station, thereby preventing the occurrence of paper jams. Thereafter, the warm-up process is completed (S26), and the display panel 92 on the operation section 90 shows that the copying operation is now available (S27), thereby completing the sequence of processes that is necessary prior to the copying operation. In contrast, in the event of a paper jam at S24, the display shows a warning message on the display panel 92 (S28), and the step S24 is repeated again. Here, at S28 a decision is made to inhibit the copying operation and to display the warning message. In the present embodiment, the arrangement as described above makes it possible to prevent jams of sheets of copy paper P in the punching unit 151 as well as preventing various troubles such as damages to the punching blade 38a. Additionally, the preventive and monitoring methods for paper jams by the use of the above-mentioned arrangement further ensure a more stable operation of the punching unit 151 if they are adopted, on demand, before and after the copying operation or between the punching operations that are successively carried out. [MODIFIED EXAMPLE] The following description will discuss a modified example of the present embodiment. In the above-mentioned embodiment, the photosensor 154 is used for detecting paper jams; whereas in this modified example, the photosensor 154 is used for judging whether or not a punching operation in question has been properly carried out. In accordance with the modified example, if light passes through a punch hole immediately after the punching operation, the judgement is made that the punching operation has been properly carried out. In contrast, if light is not allowed to pass, the judgement is made that the punching operation has not been properly carried out. In the present modified example, the CPU 155 makes the above-mentioned judgements, and it makes the punching member 38 repeat the operation (at least once) if the judgement is made that the punching operation has not been properly carried out. However, the present modified example is only applied to the arrangement where the entire portion or a punching portion of a sheet of paper P is positively stopped during the punching operation (for example, the arrangement of the punching unit 101 described in the second embodiment); it is not applied to the arrangement where the punching operation is carried out with the sheet of paper P being transported. Therefore, in order to adopt the present modified example, it is necessary to stop the transport rollers 34 and 35 temporarily during the punching operation. Referring to the flow chart of Fig. 24, the following description will discuss the operation of the copying machine in accordance with the present modified example. First, a copying operation is started (S31), and a sheet of paper P is stopped in the punching unit 151 (S32), where a punching operation is carried out (S33). Next, the photosensor 154 makes a detection as to the completion of the punching operation (S34), and if the punching operation has been completed, the transportation is resumed (S35), thereby allowing the sheet of paper P to be discharged (S36). If the punching operation has not been completed at S35, the sequence proceeds to S33. As described above, in the present modified example, if the punching operation has not been carried out properly, the punching operation is executed again; this makes it possible to prevent erroneous punching processes and improper punching processes. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention as defined in the following claims.

Claims: A paper-punching device for use in an image-forming apparatus comprising: guiding means (31) for guiding a sheet of paper (P) in a predetermined direction; a punching blade (38a) for forming a punch hole in the sheet of paper, the punching blade (38a) being installed in the guiding means; driving means (36) for driving the punching blade (38a); and transport rollers (34, 35) for carrying the sheet of paper (P), the transport rollers (34, 35) being installed on the downstream side from the punching blade (38a) in the guiding means, characterized in that the transport rollers (34, 35) being allowed to make a slip at a predetermined amount with respect to the sheet of paper (P) when the sheet of paper (P) is caught by the punching blade, the product of the time during which the sheet of paper (P) is caught by the punching blade (38a) and the transporting speed of the transport rollers (34, 35) is set at a predetermined value. The paper-punching device for use in an image-forming apparatus as defined in claim 1, further comprising: rear-edge detection means (32) for detecting the rear edge of the sheet of paper (P) on the upstream side from the punching blade; and control means (61) for activating the driving means (36) after a lapse of predetermined time from the detection of the rear edge of the sheet of paper (P) made by the rear-edge detection means. The paper-punching device for use in an image-forming apparatus as defined in claim 2, wherein the rear-edge detection means, which projects light onto a paper-transport path in the guiding means, detects the passage of the sheet of paper (P) when receiving the reflected light from the sheet of paper (P) being transported, and detects the rear edge of the sheet of paper (P) when the reflected light is no longer received. The paper-punching device for use in an image-forming apparatus as defined in claim 1, wherein the transport rollers (34, 35) are a pair of rollers, one of which is made of rubber and the other of which is made of a foamed material. The paper-punching device for use in an image-forming apparatus as defined in claim 4, the foamed material is polyurethane foam. The paper-punching device for use in an image-forming apparatus comprising as defined in claim 1, wherein the punching blades are installed in the guiding means (31) with predetermined intervals and the driving means (36) has one piezoelectric element (53) as a driving source that drives the punching blades at the same time. A paper-punching device for use in an image-forming apparatus as defined in claim 1, wherein a plurality of punching blades (38a) are installed with predetermined intervals along a straight line that tilts at a predetermined angle with respect to the direction orthogonal to the transporting direction, and a plurality of driving means (36) are installed so as to drive the punching blades (38a) individually, further comprising: a driving circuit (62) for activating the driving means (36) successively, starting with the one closest to the rear edge of the sheet of paper (P), with predetermined time-intervals. The paper-punching device for use in an image-forming apparatus as defined in claim 7, wherein the driving circuit (62) activates the driving means (36) such that the time-interval by which the driving means (36) drive the adjacent punching blades (38a) in succession is equal to a value that is obtained by dividing the product of a positional distance between the adjacent punching blades (38a) in the direction orthogonal to the transporting direction of the sheet of paper (P) and the tangent to the predetermined angle by the transporting speed of paper (P). A paper-punching device for use in an image-forming apparatus as defined in claim 1, wherein the driving means (36) has a first driving source (53) for driving the punching blade (38a) with a driving force that is suitable for normal paper having a thickness not more than a predetermined thickness and a second driving source (113) for driving the punching blade (38a) with a driving force that is suitable for thick paper exceeding the predetermined thickness, further comprising: discrimination means (109, 125) for discriminating whether the sheet of paper (P) is normal paper or thick paper; selective control means (108, 109) for activating the first driving source (53) when the discrimination means shows that the sheet of paper (P) is normal paper, while activating the second driving source (113) when the discrimination means shows that the sheet of paper (P) is thick paper; and transport control means (109) for stopping the transporting operation of the transport rollers (34, 35) during the activation of the second driving source. The paper-punching device for use in an image-forming apparatus as defined in claim 9, further comprising: thick-paper-selecting means (90, 111) for selecting thick paper as a sheet of paper (P) to be used in the image-forming operation of the image-forming apparatus, the thick-paper-selecting means (90, 111) being installed in place of the discrimination means (109, 125), wherein the selective control means (108, 109) activates the second driving source (113) when thick paper is selected by the thick-paper-selecting means (90, 111), and also activates the first driving source (53) when thick paper is not selected by the thick-paper-selecting means (90, 111). The paper-punching device for use in an image-forming apparatus as defined in claim 9, further comprising: warp-generation means (21) for allowing the sheet of paper (P) to warp at a predetermined amount; and pressure-detection means (125) for detecting a force exerted by the warp of the sheet of paper (P) that is generated by the warp-generation means (21) as a pressure, wherein the discrimination means makes a discrimination as to whether the sheet of paper (P) is thick paper or normal paper in accordance with a detection output from the pressure-detection means (125). A paper-punching device for use in an image-forming apparatus as defined in claim 1 further comprising: idle driving control means (155) for activating the driving means (36) so that the punching blade (38a) is driven in a specific period of time when there is no paper (P) at a punching position in the guiding means (31). The paper-punching device for use in an image-forming apparatus as defined in claim 12, further comprising: paper-detection means for detecting the presence or absence of paper at the punching position in the guiding means, wherein the idle-driving control means (155) refrains from activating the driving means (36) only when the presence of paper is detected at the punching position by the paper-detection means, and informs the fact that there is paper at the punching position. The paper-punching device for use in an image-forming apparatus as defined in claim 13, wherein the paper-detection means (154) is an optical sensor including a light-emitting section (154a) for emitting light that is allowed to pass through the punching position in the guiding means (31) and a light-receiving section (154b) for receiving the light from the light-emitting section (154a), the light-emitting section (154a) being disposed on the surface side of the sheet of paper (P) or on the back side thereof, the light-receiving section (154b) being disposed at a position opposite to the light-emitting section (154a) with respect to the sheet of paper (P). The paper-punching device for use in an image-forming apparatus as defined in claim 9, further comprising: punch-hole detection means (154) for detecting the fact that a punch hole is properly formed at a punching position in the sheet of maper (P) that is stopped upon operation of the second driving source (113); and re-punching control means (155) for activating the driv-ng means (72) so that at least one punching operation is carried out when no punch hole is detected by the punch-hole detection means (154). The paper-punching device for use in an image-forming apparatus as defined in claim 15, wherein: the punch-hole detection means (154) is an optical sensor including a light-emitting section (154a) for emitting light that is allowed to pass through the punching position in the guiding means (31) and a light-receiving section (154b) for receiving the light from the light-emitting section (154a), the light-emitting section (154a) being disposed on the surface side of the sheet of paper (P) or on the back side thereof, the light-receiving section (154b) being disposed at a position opposite to the lighc-emitting section (154a) with respect to the sheet of paper. The paper-punching device for use in an image-forming apparatus as defined in claim 1, wherein the punching blade (38a) catches the sheet of paper (P) for a period of time that varies depending on the thickness of the sheet of paper (P).

Inventors: HIRAI MASASHI, OKUMURA MASAFUMI, TAKA KYOUSUKE, UNO KINJI, YAMANAKA TOSHIO, YAMAUCHI HIROKAZU, YAMAUCHI YASUJI, YONEDA YOSHIHARU, HIRAI, MASASHI, OKUMURA, MASAFUMI, TAKA, KYOUSUKE, UNO, KINJI, YAMANAKA, TOSHIO, YAMAUCHI, HIROKAZU, YAMAUCHI, YASUJI, YONEDA, YOSHIHARU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1994
Main classification: B65H35
IPC codes: G03G15, B26D7, B65H35, B26D5, B26D1, B26F1